The present disclosure relates to a tomographic image generation device, a tomographic image generation method and a tomographic image generation program for generating a tomographic image from a plurality of projection images of a subject, which are obtained by imaging the subject with different radiation source positions.
In recent years, in order to more closely observe an affected part of the body with a radiographic imaging apparatus using radiation, such as x-ray or γ-ray, tomosynthesis imaging has been proposed, in which imaging is performed by applying radiation to the subject from different radiation source positions by moving the radiation source, and a tomographic image where a desired slice plane is emphasized is generated from the thus obtained projection images. In the tomosynthesis imaging, a plurality of projection images are obtained by imaging a subject with different radiation source positions by moving the radiation source in parallel with the radiation detector or along a circular or ellipsoidal arc trajectory depending on characteristics of the imaging apparatus and necessary tomographic images, and the projection images are reconstructed to generate a tomographic image using a shift-and-add method, or a back projection method such as a simple back projection method or a filtered back projection method.
However, with the tomosynthesis imaging, angles at which the radiation can be applied to the subject are limited, and, in the case where the projection images are stacked one on the other using a back projection method, for example, to reconstruct a tomographic image, the tomographic image may include artifacts which are virtual images of structures in an area of the tomographic image where the structures in the subject are not actually present. More specifically, the back projection may introduce artifacts of structures into an area of the tomographic image of a slice plane where the structures are not actually present, i.e., which slice plane is different from a slice plane where the structures are present. When such artifacts are too visible, it is difficult to see a structure, such as a lesion, which is necessary for diagnosis.
During the tomosynthesis imaging, the radiation is applied to the subject a plurality of times, and each time a radiation dose as low as possible is used to reduce the radiation exposure of the subject. However, a low radiation dose results in more quantum noise of radiation in the projection images obtained by the imaging, which in turn results in more visible noise in the reconstructed tomographic image.
Various techniques for reducing such artifacts or noise have been proposed. For example, Japanese Unexamined Patent Publication No. 2013-000261 (hereinafter, Patent Document 1) has proposed a technique which involves: calculating a similarity between each pixel of a reference projection image which is one of projection images and the corresponding pixel of each of the other projection images, which pixels are cumulatively added on the same position on a tomographic image; calculating a weighting factor for each pixel of the projection images such that the weighting factor is larger when the similarity is higher; and reconstructing a tomographic image by performing cumulative addition of products calculated by multiplying each pixel value of each projection image with the corresponding weighting factor.
Besides the above-described technique, techniques called an algebraic reconstruction method or an iterative approximation reconstruction method have also been proposed. These techniques calculate tomographic images such that images formed by projecting the reconstructed tomographic images agree with the actually taken projection images. These techniques allow incorporating various mathematical models in the reconstruction, thereby allowing taking artifact correction, noise reduction, etc., into account to generate tomographic images with suppressed artifacts and reduced noise.
Further, a technique for reducing artifacts while reducing the calculation time has been proposed, which involves: generating a plurality of bandlimited images with different frequency response characteristics from projection images; performing nonlinear transformation on the bandlimited images such that portions that exceed a predetermined value of the bandlimited images become small; adding up the bandlimited images after the nonlinear transformation to generate a plurality of transformed images; and reconstructing a tomographic image from the transformed images (see Japanese Unexamined Patent Publication No. 2013-031641, hereinafter, Patent Document 2).